Your spit is nothing like your neighbors'. A global study has found vast differences between people in the types of microbes inhabiting their saliva--with a surprising lack of correlation between the type of bugs and where a person lives.
Until recently, biomedically oriented microbiologists tended to focus on just those pathogens that could be grown in the laboratory. But in the past decade, ever-cheaper sequencing technology has enabled wholesale surveys of genes in the environment, and from those genes, assessments of entire communities of microbes associated with the gut and other parts of the body (Science, 2 June 2006, p. 1355). Certain community compositions are proving to be risk factors, say, in obesity. In 2007, the U.S. National Institutes of Health set up a $115 million, 5-year program to look in-depth at microbes found on or in people.
But Mark Stoneking wasn't all that interested in human health when he decided to chase down the microbes in spit. A molecular anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, he and colleagues wanted to know whether cheek swabs or saliva were better sources of human DNA to use in his studies of human migrations. They did some tests. "You do get a whopping lot of DNA out of saliva, but much of it turns out to be bacteria," he recalls.
That got him thinking. He had been impressed by how anthropologists were able to trace human migrations through the differences in the strains of the stomach bug Helicobacter pylori in various groups of people (Science, 7 March 2003, p. 1582 ; Science, 23 January, p. 527). "With that species, you see very strong geographic patterning," says Stoneking. But getting a sample of H. pylori is relatively difficult, as it requires a stomach biopsy. He wondered whether any of the bacteria in spit would work instead.
Stoneking persuaded colleagues to help collect saliva from 10 volunteers each at a dozen locations around the world. His lab isolated and sequenced about 500 bases from a gene often used to tell one type of bacterium from another. They collected about 120 sequences of this gene from each sample. They then matched those sequences against those in public databases to identify each individual's bacteria.
Taken together, the samples contained 101 known bacterial genera, 39 of them newly found in the mouth, the team reports today in Genome Research. The samples also included another estimated 64 genera that were not in the databases. Individuals had between six and 30 kinds of bacteria in their mouths. The 10 individuals from California and the 10 from the Congo had the greatest variation; those from the Republic of Georgia and Turkey had the least.
Differences in the bacterial composition among individuals from a particular place tended to be on par with the differences in the bacteria present between individuals from different places, indicating--to the team's surprise--that there were no geographical patterns. The researchers also did not find any correlation between the bacteria present and age, gender, or environmental factors, such as rainfall. One might expect diet or hygiene to affect the microbial communities, but the researchers did not assess those cultural factors in this particular study. Thus, they have few clues as to why this diversity exists.
"These communities are very complex, and the degree to which they vary between people continues to surprise [us]," says Ruth Ley, a microbial ecologist at Cornell University. She would like to see the researchers follow up the study by looking at more people, taking multiple samples over time from each person, and getting more sequence from each sample. Such efforts may prove worthwhile for more than just anthropological questions, says Daniel Falush, a statistical geneticist at the University of Oxford in the United Kingdom: "Variation in this microbiota may reveal unexpected insights into many aspects of health."